1. Field of the Invention
The present invention relates to a back light unit, and more particularly, to a back light unit that improves light condensing efficiency and viewing angle characteristics.
2. Discussion of the Related Art
Recently, various flat panel displays that can reduce weight and volume of a cathode ray tube have been developed. Examples of the flat panel displays include a liquid crystal display (LCD), a field emission display (FED), a plasma display panel (PDP), and a light emitting display (LED).
Among them, the LCD displays desired images by controlling light emitted from a back light unit using an LCD panel. The LCD panel includes a plurality of liquid crystal cells and a plurality of control switches for switching video signals to be supplied to the respective liquid crystal cells.
FIG. 1 illustrates a related art back light unit.
As shown in FIG. 1, the related art back light unit includes a lamp 10 generating light, a light guide plate 20 acting as a surface light source by dispersion of the light generated from the lamp 10 that enters through an incident surface 22, a lamp housing 12 arranged to surround the incident surface 22 of the light guide plate 20 and the lamp 10, a reflecting plate 30 arranged below the light guide plate 20, a diffusion sheet 40 arranged above the light guide plate 20 to diffuse the light passing through the light guide plate 20, and first and second prisms 50 and 60 controlling the direction of the light passing through the diffusion sheet 40.
Generally, the lamp 10 is formed of a cold cathode fluorescent lamp. The lamp 10 is driven by a lamp driving voltage from an inverter (not shown) and emits light to the incident surface 22 at a side of the light guide plate 20.
The lamp housing 12 is arranged at the side of the light guide plate 20 to surround the lamp 10 and the incident surface 22 of the light guide plate 20.
The lamp housing 12 has a reflecting surface therein to reflect the light from the lamp 10 toward the incident surface 22 of the light guide plate 20.
The light guide plate 20 allows the incident light from the lamp 10 to reach a portion away from the lamp 10 and guides the incident light to the diffusion sheet 40.
In other words, a printing pattern is formed on a tilted rear surface of the light guide plate 20 to reflect the light from the incident surface 22 at a predetermined tilt angle from the tilted rear surface and uniformly progress the reflected light to the diffusion sheet 40.
The reflecting plate 30 is arranged below the light guide plate 20 to reflect again the light entering through the rear surface of the light guide plate 20 to the light guide plate 20 so as to reduce light loss.
The diffusion sheet 40 diffuses the light passing through the light guide plate 20 into entire regions and irradiates the light to the first prism sheet 50.
Meanwhile, light incident to a liquid crystal panel (not shown) has great light efficiency when the light vertically enters the liquid crystal panel. In this respect, two forward prism sheets are preferably deposited so that the light emitted from the light guide plate 20 is vertical to the liquid crystal panel.
The first and second prism sheets 50 and 60 serve to condense the light passing through the diffusion sheet 40.
For this, each of the first and second prism sheets 50 and 60, as shown in FIG. 2, includes a condensing film 52 of polyester(PET) and a plurality of prism peaks 54 formed on the condensing film 52 in a stripe shape.
The prism peaks 54 have first and second tilt surfaces tilted from their angular points at a predetermined angle. At this time, each of the first and second tilt surfaces is tilted from a top surface of the condensing film 52 at an angle of 45°.
The incident light with a predetermined angle θ1 to the first and second prism sheets 50 and 60 having a refractive index n1 is refracted by the first and second prism sheets 50 and 60 at a predetermined angle θ2 under the Snell's law expressed in the following equation 1 and then emitted to the outside having a refractive index n2.
                                          n            ⁢                                                  ⁢            1                                n            ⁢                                                  ⁢            2                          =                              sin            ⁢                                                  ⁢            θ1                                sin            ⁢                                                  ⁢            θ2                                              equation        ⁢                                  ⁢        1            
In the aforementioned related art back light unit, the light emitted from the lamp 10 proceeds to the diffusion sheet 40 arranged above the light guide plate 20 through the light guide plate 20, and the light passing through the light guide plate 20 is diffused to the entire regions through the diffusion sheet 40. Then, the diffused light is condensed through the first and second prism sheets 50 and 60. The condensed light is finally emitted to the outside.
However, in the related art back light unit, the incident light to the first and second prism sheets 50 and 60 can be split into three regions, as shown in FIG. 3, i.e., a total reflection region, a condensing region, and a side lobe region.
In more detail, light A of the total reflection region vertically enters the condensing film 52 and is totally reflected by the first and second tilt surfaces of the prism peaks 54. The totally reflected light proceeds again to the light guide plate 20. In this way, the light A is condensed by recycling.
Light B of the condensing region enters the condensing film at a predetermined angle and is condensed in such a manner that it is refracted by the first and second tilt surfaces of the prism peaks 54.
Light C of the side lobe region enters the condensing film 52 at a predetermined angle and is totally reflected by the first and second tilt surfaces of the prism peaks 54. In this case, light efficiency and viewing angle characteristics are deteriorated.
The first and second prism sheets 50 and 60 have a better condensing efficiency in a vertical direction (Y axis) than a condensing efficiency in a horizontal direction (X axis) as shown in FIG. 4 due to a two-dimensional structure of the prism peaks 54.
Therefore, as shown in FIG. 5, a bright region exists at both sides of each of the first and second prism sheets 50 and 60 around a symmetrical point due to the side lobe.
As a result, the related art back light unit has some problems due to the structure of the first and second prism sheets 50 and 60. That is, the viewing angle characteristics deteriorated due to luminance asymmetry in the vertical and horizontal direction (Y and X axes), and condensing efficiency deteriorates due to the side lobe.